Article pubs.acs.org/JPCB
Influence of Molecular Structure on the Size, Shape, and Nanostructure of Nonionic CnEm Surfactant Micelles
Faheem N. Padia,† Mohammed Yaseen,† Barbara Gore,‡ Sarah Rogers,§ Gordon Bell,∥ and Jian R. Lu*,† †
Biological Physics Laboratory, School of Physics and Astronomy, University of Manchester, Manchester, M13 9PL, U.K. School of Chemistry, University of Manchester, Manchester, M13 9PL, U.K. § STFC ISIS Facility, Rutherford Appleton Laboratory, Didcot OX11 0QX, U.K. ∥ Syngenta, Jealott’s Hill International Research Centre, Bracknell, Berkshire RG42 6EY, U.K. ‡
ABSTRACT: Nonionic alkyl ethoxylates (CnEm) have been extensively studied for their adsorption, aggregation, and solubilization individually and in small groups. In this work, we report a more systematic study of the effects of alkyl chain (tail) and ethoxylate (head) length on the size, shape, and extent of intermixing within the CnEm micelles in aqueous solution. Data from small angle neutron scattering (SANS) and nuclear magnetic resonance (NMR) were combined to undertake the structural characterization of micelles formed from the two separate series of surfactants CnE6 (n = 10, 12, 14) and C12Em (m = 5, 6, 8, 10, 12). The micellar core volume (Vcore) could be well determined with reasonable accuracy and linked to the hydrophilic−lipophilic balance (HLB) of the surfactant, with a sharp size and shape transition occurring around HLB = 12.5. NOESY NMR results revealed protrusions of the terminal methylene groups into the ethoxylate shell, thus providing direct experimental evidence for the phenomenon of “roughness” or intermixing of the core−shell interface. These detailed studies are compared with previous investigations on this model surfactant system.
1. INTRODUCTION There is continuing interest in the relationship between surfactant molecular structure and micellar structure. Such knowledge is fundamental not only for the development of new surfactant based drug delivery systems1,2 and aqueous agrisprays3 but also for the improvement of many existing surfactant based products including cosmetic formulations and cleaning detergents. Insights into the relationship between surfactant molecular structure and micellar structure have already been gained by previous studies. For example, Puvvada and Blankschtein showed that a surfactant’s hydrophobic tail structure has a greater influence on micellar structure than its hydrophilic portion by systematically evaluating the thermodynamic contrbutions from all species involved in micellization.4 With the exception of some early experimental studies on the aggregation numbers of alkyl ethoxylate (CnEm) micelles5,6 and recent studies of their structures by small angle neutron scattering (SANS) for distinct non-ionic surfactants such as C12E6 and C12E12,9,11−13 experimental data to support theoretical findings is generally lacking. Information about the transition of the size and shape of such non-ionic surfactants with alkyl chain length and the size of the ethoxylate head deserves further investigation. Such work can help develop a more quantitative relationship between the molecular structure for the non-ionic series and the micellar structure to be formed. In this work, we have investigated the effects of the alkyl chain length (tail, n) and the number of ethylene glycol units (head, m) on the transition of the size and shape of the non-ionic © 2013 American Chemical Society
micelles using SANS in a more systematic manner. In addition, proton NMR (1H NMR) was used to unravel the detailed hydration of the ethoxylate units and two-dimensional nuclear Overhauser effect spectroscopy (2D NOESY) revealed the conformation of alkyl chains and the spatial proximity of pairs of coupled protons providing useful insight of possible intermixing between alkyl tail segments and the ethoxylate units across the curved interface. These non-ionic surfactants were ideal for the purpose of this study because the lengths of the head and tail groups could be varied independently so that their respective influences on micellar structure could be determined. Moreover, they are among the most commonly used in research and serve as useful models for commercial non-ionic surfactants widely used in industry. Micellization of non-ionic surfactants has been extensively studied to account for the effects of concentration,7 temperature,8 and electrolyte,9 and the influences of addition of surface active additives and mixing with another surfactant.10−13 Although the basic surface packing theory provides a useful outline of micellar shape transition on the basis of surfactant structure, the actual situation is often complex due to the different interactions involved, making it difficult to draw reliable conclusions. For example, Mitchell et al. inferred micellar shapes for a series of CnEm surfactants from macroscopic phase measurements.14 Although the micellar Received: October 2, 2013 Revised: December 2, 2013 Published: December 4, 2013 179
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the same micelles are all oblate or “disk-like” on the basis of sedimentation velocity and intrinsic viscosity data.23 This difference and the model dependent uncertainties inherent of the scattering data analysis suggest that even with the modifications applied to the core−shell model it still may not suitably describe the structure of CnEm micelles. This process must thus be restricted by the sensitivity of the SANS data fitting to the extent of intermixing. To complement the SANS studies in this work, we chose to use an alternative model in which no assumptions are made at the start about the hydration of the head groups and intermixing of the head and tail. The outcome will allow some direct comparison with the previous SANS findings. We have adopted the model developed by Pedersen and Gerstenberg24 and treated the ethoxylate (EO) head groups as individual chains as opposed to a homogenously hydrated mixture.25,26 We will thus need to compare our outcome with the work by Penfold et al. and others to justify the validity of such treatment. With the practical implications of this study to industrial significance, the data is presented with respect to the hydrophile−lipophile balance (HLB), a concept originally introduced by Griffin to correlate the characteristic solution and interfacial properties of non-ionic CnEm surfactants to their molecular structure.27 It was defined empirically and is expressed as
shape predictions were based on a sound geometric-packing theory, two assumptions about the hard-core repulsion between micelles and the order/disorder transition from the isotropic micellar phase to the high concentration phase were difficult to justify. Neutron scattering work by Zulauf et al. later put direct experimental evidence into these systems and showed that these assumptions were not necessarily correct.15 Techniques such as small angle neutron and X-ray scattering (SANS16 and SAXS), dynamic light scattering (DLS),17 and NMR18 provide direct experimental information about different aspects of micellar structures. The scattering profiles from SAXS and SANS contain key structural details about the micelles, but structural parameters about micellar size and shape must be extracted from model analysis or data fitting.19 Although this is a widely adopted and well exercised approach, two or more models may fit the same scattering profile, resulting in different sets of parameters and hence uncertainties of the true micellar structure. In comparison with SAXS, the situation for SANS can be worse because of the narrower momentum transfer range over which data can be reliably determined. However, SANS in combination with deuterium labeling is effective at resolving micellar structures because several scattering profiles can be measured about the same chemical system, thereby providing better structural constraints. Different models have been developed to analyze scattering profiles measured from micellar aggregates. However, they are all based on the common “core−shell” feature with adjustments to account for size and shape deviations. In the representative model for non-ionic CnEm, the core of the micelle is regarded as the homogeneous droplet of the pure alkyl chains and the outer shell contains the respective hydrated head groups. Thus, the whole micelle can be defined from just three parameters, R1, R2, and e, denoting the core radius (which is often constrained to be less than of equal to the length of the fully extended chain), the core + shell radius, and the eccentricity (accounting for the ellipticity of the micellar shape and is equal to R1/R2), respectively. Similar models are widely used to fit SANS data; however, experimental and molecular simulation studies have shown that some of the assumptions made above might not be correct. For example, it is often assumed in the model that there is no intermixing between the core and shell regions but Lu et al. showed from neutron reflection studies on a monolayer of C12E12 that there is a region of intermixing between the alkyl chains and ethoxylate head groups and that the region extends to around 35% of the length of the alkyl chain.20 It is also assumed that the ethoxylate shell is homogenously hydrated but Podo et al. and Majhi et al. showed using 1H NMR21 and single-chain mean field theory,22 respectively, that different extents of hydration existed in the shells of various CnEm micelles. It is often difficult to assess the impact of any incorrect assumptions on the data analysis due to the multiple parameter features, but the core−shell model in its most basic form and modified version does need to be tested and enriched with further structural information from the nonionic surfactant micelles. A number of researchers have overcome the shortcomings of the core−shell model by making certain modifications.13−15 For example, Penfold et al. introduced an additional parameter, ext, to allow for variations in micellar core volumes. The introduction of this new parameter enabled them to fit experimental data for C12E6, C12E8, and C12E12 micelles which were all shown to have prolate or “rod-like” shapes and have advanced from the findings of Tanford et al. who suggested that
HLB =
%mass EO 5
(1)
where %massEO is the mass percentage of the EO head. According to eq 1, the HLB number ranges from 1 to 20 with increasing HLB representing more hydrophilic surfactants. The empirical definition and strong correlation of HLB with surfactant properties has led to HLB becoming one of the most commonly used indicators in industry for determining the suitability of non-ionic surfactants in a given application. In a recent study, Diallo et al. demonstrated that HLB was also a convenient way of systematically ordering a group of surfactants with regard to their micellar properties.28 In this work, we combined data obtained from SANS to elucidate the surfactant molecular structure−micellar structure relationship and NMR to investigate the hydration and intermixing within the micelles.
2. EXPERIMENTAL SECTION 2.1. Materials. Two series of surfactants were purchased from Sigma-Aldrich, CnE6 (where n = 10, 12, and 14) and C12Em (where m = 5, 6, 8, and 10) (≥98%), and were used without further purification. Fully hydrogenated hC12hE12 and chain deuterated dC12hE12 were provided by Dr RK Thomas at University of Oxford. hC12hE12 was prepared by reacting hC12hE6 with hexaethylene glycol (E6) (Fluka, >98%), following the procedures described previously.20 Dried hC12hE6 was first reacted with p-toluenesulphonyl chloride (Fluka, 99%) under dry triethylamine (Aldrich, 99%) to obtain C12E6-tosylate. In a separate flask, dried EO6 in excess was gently mixed with potassium tert-butoxide (Aldrich, 99%) to obtain K-EO6. When the reaction was complete, the sample was mixed with the C12E6-tosylate. The mixture was stirred and heated at 70 °C for 2 h, followed by the addition of 10 mL of hot water to stop the reaction. When cooled, the mixture was neutralized with HCl and extracted with ether. Following solvent evaporation, the raw sample (light yellow oil) was purified through silica flash chromatography. Alkyl chain 180
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S(Q) is the structure factor taking into account the scattering between different particles in the sample, and B is the background signal. For a given sample, the first three terms are constants and in each measurement the background (solvent) signal is reduced from I(Q). Therefore, P(Q) and S(Q) are the only Q-dependent variables. In essence, P(Q) and S(Q) modulate the intensity profile I(Q); thus, I(Q) holds information about all physical parameters of the system. For a dilute system of non-interacting particles such as a low concentration non-ionic micellar solution S(Q) is close to unity. Equation 2 thus simplifies so that I(Q) becomes ∝ P(Q). It thus becomes more straightforward to determine the form (shape and size) of the scattering particle by fitting I(Q). In resemblance to the model developed by Pedersen and Gerstenberg,24 the micellar core is treated as a homogeneous oil droplet consisting of the alkyl chains and the head groups comprised of ethoxylate units adopting the Gaussian chain distribution. Unlike the typical core−shell models used, no assumptions about the hydration of the head groups and intermixing of head and tail groups are made prior to the start of the fitting, as these issues can be easily accommodated by incorporating methylene groups from the tail into the head shell or ethoxylate groups into the tail core during the fitting process. The treatment of short ethoxylate groups as a Gaussian distribution lends its support from the work of Sarmoria and Blanckshtein who showed that even relatively short ethoxylate chains follow standard polymer scaling laws.35 Pedersen and Gerstenberg have derived a number of models for different core structures, but we chose the rotational shape with semiaxes (a, a, c),36 as this treatment was found to be suitable for the CnEm micelles under study and helped reduce the number of parameters used. The model is further outlined below. I(Q) is split into four form factor contributions consisting of the core PC(Q), a single headgroup PH(Q), the interference between head and core PHC(Q), and that between head groups, PHH(Q):
deuterated dC12hE12 was synthesized following a similar procedure as described above. dC12hE6 was made by the Williamson reaction from the deuterated dodecyl bromide, an equimolar amount of sodium, and a 5-fold molar excess of E6.29 All non-ionic surfactants, self-prepared or directly purchased from Sigma-Aldrich, showed single clear spots from our own thin film chromatography (tlc) analysis under a range of concentrations loaded, showing no sign of chain or head mixing. Surfactant solutions were prepared at 100 times their CMCs (critical micellar concentrations) by dissolving the required volume/mass of surfactant in 1 mL of D2O (containing 99.9% D from Sigma-Aldrich). The following CMC values were used:30 0.90 mM for C10E6, 0.067 mM for C12E6, 0.010 mM for C14E6, 0.062 mM for C12E5, 0.10 mM for C12E8, and 0.10 mM for C12E12. For NMR experiments, TSP (3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt) was added to the micellar solutions at a concentration of 1% w/v as a measurement standard. 2.2. Experimental Methods. SANS experiments were conducted on the time-of-flight LOQ diffractometer at the ISIS neutron facility, and the Q-range of 0.009−0.249 Å−1 was used in data analysis. Standard procedures for data treatment were employed.31 Measurements were performed in a 2 mm quartz cell, using a 12 mm diameter beam. The temperature for all experiments was kept constant at 295 K to ensure that samples remained in a single phase throughout the scattering experiment. After data collection completed, the D2O (solvent) scattering profiles were subtracted from beam intensities for background correction. Dynamic light scattering (DLS) was also widely used to study surfactant aggregation.32 This technique helped reveal the hydrodynamic radius, RH, of a scatterer and complements SANS. DLS measurements were performed on a Malvern Zetasizer Nano instrument. Surfactant samples were loaded in quartz cells and left to equilibrate for 5−10 min prior to the measurements. For each sample, three measurements were taken consisting of 15 runs each. Each run lasted 5 s. Values for RH of the micelles were calculated by averaging the three measurements. The temperature for all measurements was kept constant at 295 K. NOESY is highly effective at investigating the internal structure of micelles,33 while 1H NMR can help reveal the extent of hydration of alkyl chain fragments within micelles.34 The 1H NMR and NOESY spectra were recorded in D2O on a Bruker Avance 400 MHz spectrometer. Each spectrum consisted of 16 scans. The gradient NOESY spectra were recorded using a mixing time of 300 ms, and a relaxation delay of 2s and 2k data points was collected for 512 increments of 32 scans. 2.3. SANS Data Analysis. The scattered intensity from a monodisperse sample can be approximated as 2
2
I(Q ) = NV Δρ P(Q )S(Q ) + B
I(Q ) = Nagg 2βC 2PC(Q ) + NaggβH 2PH(Q ) + 2Nagg 2βHβCPHC(Q ) + Nagg(Nagg − 1)βHH 2PHH(Q )
(3)
where Nagg is the aggregation number of the micelle; βC and βH are equal to VC(ρC − ρsolv) and VH(ρH − ρsolv), respectively; Vi and ρi are the volume and scattering length density of an individual chain, respectively; C denotes the hydrophobic core and H denotes the hydrophilic head; and ρsolv is the scattering length density of the solvent, i.e., D2O. Final form factors for PC(Q), PH(Q), PHC(Q), and PHH(Q) are given in eqs 4−8. PC(Q ) =
(2)
∫0
π /2
Φ2[QR(a , ε , α)] sin α dα
(4)
where R(a, ε, α) = r(sin2 α + ε2 cos2 α)1/2, ε = c/a, and
where Q is the momentum transfer, N is the number concentration of the scattering particles, V is the volume of a single scattering particle, and Δρ is the contrast of the scattering particle and is equal to the difference between the scattering length densities of the particle and the solvent, ρs and ρsolv, respectively. P(Q) is the form factor of the scattering particle which introduces interference effects from neutrons scattered from different parts of the same scattering particle,
Φ(QR ) =
3[sin(QR ) − QR cos(QR )] (QR )3
(5)
PH(Q ) =
2[e−x − 1 + x] x2
(6)
where x = Q2Rg2. 181
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Figure 1. (a) SANS intensity profiles measured from C12Em (m = 5 (○), 6 (△), 8 (□), and 12 (◇)) at 100CMC in D2O, with points representing experimental data and curves representing the best fits. (b) For C12E6, the two best fitted curves for different Q regions are given: the dashed curve corresponds to C12E6a and the solid curve to C12E6b as shown in Table 1 to suggest the coexistence of different micellar sizes.
Table 1. Structural Parameters Obtained from the Best Fits Shown in Figure 1 for C12Em (m = 5, 6, 8, and 12) SANS
DLS
surfactant
a (nm ± 0.1 nm)
c (nm ± 0.1 nm)
Vcore (nm )
Nagg
Rg (nm ± 0.05 nm)
Rmax (nm)
RH (nm ± 0.2 nm)
PDI
C12E5 C12E6a C12E6b C12E8 C12E12
1.00 1.50 2.80 1.8 2.0
55.00 8.00 1.25 1.25 1.20
230.4 75.4 46.5 26.5 20.1
710 233 141 80 65
0.45 0.55 0.55 0.60 0.65
55.9 9.1 4.9 3.5 3.3
32.1 5.3 5.3 4.4 4.6
0.231 0.332 0.332 0.101 0.111
3
In the following equations, the short notation R = (R(r, ε, α) will be used for convenience. PHC(Q ) = ψ (QR g)
∫0
π /2
Φ(QR )
sin(Q [R + dR g ]) Q [R + dR g ]
low Q and mid-high Q regions, as illustrated in Figure 1b. This indicates that the scattering profiles can be represented by a single shape of micelles in C12E5, C12E8, and C12E12 solutions, whereas the C12E6 solution may consist of more than one micellar shape. This is discussed in more detail below. Table 1 lists all the fitted parameters for the CnEm micelles studied. The dimensions of the micellar cores are given by the values for a and c. In order to ensure no void existed in the center of the liquid micelle, a was constrained to be ≤ lmax (1.67 nm)the length of the fully extended alkyl chain. For C12E5, C12E8, and C12E12, it can be seen that the a values follow this requirement well, consistent with Tanford’s proposal37 that the shortest dimension of the micellar core is close to 0.8 × lmax ′ , where l′max is the length of the fully extended alkyl chain composed of (n − 1) methylene groups. Tanford proposed that the CH2 group closest to the ethoxylate headgroup protrudes into the shell leaving (n − 1) CH2 groups in the core adopting a conformation with its total length being just shorter than its fully extended length. For a C12 chain, the shortest dimension would, therefore, be 1.22 nm, following Tanford’s estimate. The other dimension, c, was not constrained and was freely fitted. It can be seen that while a remains fairly constant there is a sharp reduction in c with increasing m. In terms of micellar shape, these dimensional changes translate to a shape transformation from prolate ellipsoid for C12E5 to oblate ellipsoid for C12E8. Thus, by increasing the length of headgroup by three ethoxylate units, the micellar cores underwent a prolate-to-oblate shape transformation. For m > 8, there is little further change in micellar shape but there is a slight reduction in the micellar core volume, Vcore. It is useful to comment further on the transformation on the shape of C12E6 micelles. According to the two best fit curves (Figure 1b), two different shapes of micelles, prolate and oblate ellipsoids, coexisted in the C12E6 solution. As the SANS scattering profile represents the average scattering from all species within a sample over an extended period of time
sin α dα (7)
2
PHH(Q ) = ψ (QR g)
∫0
π /2
⎡ sin(Q [R + dR g ] ⎤2 ⎢ ⎥ sin α dα ⎢⎣ Q [R + dR g ] ⎥⎦ (8)
where ψ(x) = [1 − e−x]/x and d ≈ 1, ensuring that the head chains did not protrude into the core since the head chains and core were modeled as two unique structures. This did not imply that intermixing of head and tail groups was not possible. Instead, d was used purely for the mathematical consistency of the model. Overall, the model depends on three fit parameters, a, c, and Rg, where a and c are the semiminor and semimajor axes of the elliptical micelle, respectively, and Rg is the effective radius of gyration of a headgroup chain. Thus, it had fewer fitting parameters than the most core−shell models currently used.
3. RESULTS AND DISCUSSION 3.1. Micellar Nanostructure. 3.1.1. Head Length Effect. SANS scattering results for the surfactant series C12Em (m = 5, 6, 8, and 12) are shown in Figure 1a. For clarity, the SANS profiles are translated in the vertical axis. It can be seen from Figure 1a that there are clear changes in the shape of the scattering profiles as m increases, indicating changes in micellar size and shape. The close fit of the experimental data profiles to the simulated curves achieved using the rotational model as described previously suggests that all four surfactants form ellipsoidal micelles. For C12E5, C12E8, and C12E12, a single set of parameters gave a suitable fit to all the data points, but for C12E6, two separate sets of parameters were necessary to fit the 182
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Table 2. Structural Micellar Parameters Obtained from the Best Fits Shown in Figure 3 for CnE6 (n = 10, 12, and 14) SANS
DLS
surfactant
a (nm ± 0.1 nm)
c (nm ± 0.1 nm)
Vcore (nm3)
Nagg
Rg (nm ± 0.05 nm)
Rmax (nm)
RH (nm ± 0.2 nm)
PDI
C10E6 C12E6b C12E6a C14E6
2.25 2.80 1.50 1.50
1.00 1.25 8.00 20.00
21.2 46.5 75.4 188.5
80 141 233 500
0.55 0.55 0.55 0.55
3.4 4.9 9.1 21.1
3.4 5.3 5.3 18.8
0.066 0.332 0.332 0.207
C12E12, we verify the model analysis based on the ellipsoidal form factor approach by first deriving micellar shape changes following a consistent trend as outlined in Table 1 and second by fitting the derived micellar parameters for C12E12 to a second set of SANS profiles measured under different isotopic contrasts. Figure 2 shows the experimental SANS scattering profiles of the fully protonated C12E12 (isotopic composition hC12hE12)
(approximately 1 h), the data indicates that the two groups of micellar populations are in equilibrium. The possible coexistence of different but similar micellar shapes has also been suggested by Gapinski et al. They commented that while they could use the elliptical rod-like structure as an overall representative model, their data could also be well analyzed using a similar approach. The coexistence of different micellar shapes is actually not new, as Nagarajan had predicted the coexistence of micellar populations over a decade ago.38 However, only recently has there been evidence to support his claim. For example, simulations performed with the recently developed single-chain mean field theory suggest that coexistence of multiple populations is quite common.22,39 Work by Glatter et al.40 and another study by BernheimGroswasser et al.41 using cryo-TEM imaging also led to evidence for coexisting micellar populations, showing spherical and thread-like micelles from their non-ionic surfactant solutions. The polydispersity index values, PDI, obtained from DLS measurements also support these observations. The PDI indicates the distribution of correlation functions (which are used to calculate RH) measured in a sample.42 For a monodisperse sample, the distribution of correlation functions is small and hence the PDI value is small. On the contrary, a polydisperse sample will suffer from a large distribution resulting in a high PDI number. As shown in Table 1, the PDI numbers for C12E8 and C12E12 are small, indicating that these micelles are rather monodisperse. For C12E5 and C12E6, however, the PDI numbers are much larger, suggesting a higher degree of polydispersity associated with different sizes or dimensions in the micellar systems. The volume of the micellar core, Vcore, was calculated from the core dimensions for each micelle. It can be seen that the shape transformation is accompanied with an approximately 10fold reduction in volume. Additionally, there is a small increase in Rg with m, which suggests that, in spite of the large transformation of core shape and size as a result of the growth of the headgroup, the conformation of the headgroup does not change significantly. Nagg, the average micellar aggregation number was calculated by dividing Vcore by Vchain, the volume of a single hydrocarbon chain, as given by Tanford’s expression Vchain = 27.4 + 26.9(nc − 1)
Figure 2. SANS scattering profiles for C12E12 at 100CMC under different isotopic contrasts, hC12hE12 (◇) and dC12hE12 (×). Points represent experimental data, and curves represent simulated best fit curves. The same structural parameters fit both sets of experimental data, adding confidence to the model proposed.
and chain deuterated C12E12 (dC12hE12) as an example, with the calculated curves of the best fits also shown. The best fit curves are produced using the same ellipsoidal model and parameters (see Table 1) for both isotopic contrasts. As the isotopic composition of the surfactant has no effect on the micellar nanostructure but does change the scattering profiles, the close fit of the calculated curves to the experimental data as illustrated in Figure 2 verifies the ellipsoidal model and the obtained parameters. The hydrodynamic diameter, RH, was also measured for each micellar solution using DLS, with the measured values listed in Table 1. In DLS measurements, RH is calculated from the diffusion constant, D, of a particle using the Stokes−Einstein relation which is given by
D=
(9)
where nc is the number of carbons in the chain. It can be seen that Nagg decreases gradually with m. This is in agreement with previous experimental data that showed an almost linear decrease in aggregation number with head length (see Tables 1 and 2).5,6 The Nagg values for C12E6b and C12E8 are 141 and 80, respectively, which is in close agreement with those reported by Zulauf et al., 140 and 95, respectively.15 On the other hand, the Nagg value of 65 for C12E12 is lower than that of 79 reported by Penfold et al.13 It is useful to check the reliability of the Nagg values calculated so that the discrepancies could be tested. For
kT 6πηRH
(10)
where k is the Boltzmann constant, T is the temperature in K, and η is the viscosity of the solvent. The relation in eq 10 is only true for spherical particles. For non-spherical particles, the value of RH does not relate to the actual radius of the particle, but instead it gives a value that is equivalent to the radius of a spherical particle that would diffuse with the same diffusion constant as the scattering particle in the sample. As such, the measured RH value is only used here as an indication of the order of magnitude of the micellar dimensions. It can be seen that RH is similar to Rmax (the biggest dimension obtained from 183
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C12E5) or whether this is due to the dynamic nature of micelles which inherently leads to greater polydispersity in larger micelles. The RH values for all the CnE6 surfactants are of the same order of magnitude as Rmax, thus broadly consistent with the dimensions obtained from SANS modeling. 3.2. Hydrophile−Lipophile Balance (HLB). As HLB is widely used as a measure of changes in amphiphilicity, we also examined how the shape and size changes were affected by it. This is done using the HLB, as shown in Figure 4, where
SANS) for all C12Em micelles, consistent with the physical implication of the hydrodynamic dimension. 3.1.2. Chain Length Effect. Changes in the scattering profiles as a result of increasing the alkyl chain length for CnE6 (n = 10, 12, and 14) are shown in Figure 3, with an isotopic composition of hC12hEm in D2O.
Figure 3. SANS intensity profiles for CnE6 (n = 10 (◇), 12 (□), 14 (△)) micelles at 100 times cmc in D2O. Points represent experimental data, and curves represent the best fits using the triaxial model. For C12E6, the two best fitted curves to different Q regions are given: the solid curve corresponds to C12E6a in Table 1, and the dashed curve is C12E6b.
Figure 4. Changes in micellar core volume with HLB for the C12Em series (◆) and the CnE6 series (●), with shape transitions also schematically represented. There is a sharp drop in micellar core volume around HLB = 12.5 where different shapes of micelles might coexist.
The micellar parameters obtained from the data fit are listed in Table 2. The two sets of fitted parameters for C12E6 have been included in reverse order in Table 2 to illustrate the shape transformation with increase in n. The transformation is opposite to that occurring when m is increased. When n = 10, the micelles are small and oblate, but for n = 14, the micelles are much longer and prolate in shape. The overall shape transformation is thus oblate-to-prolate when n increases. Although this shape transformation is predicable, it has not been shown for these alkyl ethoxylate micelles. A similar transformation was observed by Arleth et al. in a mixed system of egg yolk phosphatidylcholine (PC) and polyethylene glycol (PEG) modified distearoyl phosphatidylethanolamine (DSPEPEG).43 DSPEPEG had a bulkier headgroup, and the mixed micelles were found to transform from oblate to prolate ellipsoid as the mole fraction of PC increased from 0 to 0.4. Thus, the preference for prolate micelles when the hydrophobicity of the surfactant(s) is increased is consistent in both systems. Table 2 also shows that Rg is constant with n, suggesting that there was little conformational change of the head groups as the alkyl chain was increased. In the previous section, we saw about 10-fold reduction in core volume when the number of ethoxylate units in the headgroup was increased from m = 5 to 12. In contrast, Table 2 shows nearly a 10-fold increase in core volume as the chain length is increased from n = 10 to 14. Despite the similar magnitude of the volume change, the relatively small change in tail length has had a much greater influence on the micellar core volume than the head length, thus supporting the theoretical prediction by Puvvada and Blankschtein.4 Table 2 shows a similar trend in PDI values as seen in the previous section. The small oblate C10E6 micelles show low polydispersity, while C12E6 exhibits a much higher PDI value due to the coexistence of multiple micellar populations. It is not clear from this data, however, whether the high PDI value for C14E6 is also due to coexistence of micellar populations (as with
micellar core volumes are plotted against HLB with the corresponding micellar shapes overlaid. The squares represent surfactants in the C12Em series, and the diamonds denote the CnE6 series. The results shown in Figure 4 indicate a rapid reduction in micellar volume as HLB increases when HLB < 12. The reduction in volume is accompanied by a prolate-to-oblate shape transformation. At around HLB = 13, changes in size and shape plateau and further increase in HLB has no effect on micellar shape, although a slight reduction in volume occurs, demonstrating a general relationship for the non-ionic surfactants. From Figure 4, it can be seen that the coexistence of different micellar shapes in C12E6 coincides with the shape and size transition at around HLB = 12.5. To alleviate any potential concerns that the presence of the two shapes arose due to impurities in the surfactant, thin layer chromatography and NMR were both performed on the C12E6 samples. No impurities were found. 3.3. Shell Structure and Hydration. To complement the SANS study, we also examined the internal structure of the micelle and the hydration of the micellar shell using 1H NMR and two-dimensional nuclear Overhauser effect spectroscopy (2D NOESY). 3.3.1. 1H NMR. The chemical shifts of protons in the C12E6 micellar solution are shown in Figure 5. The contribution to the NMR spectrum from monomers is assumed to be negligible as the sample concentration was much greater than the CMC. The peaks are identified as follows: 0.78 ppm - terminal methyl group in alkyl chain, 1.2 ppm - C9H18 bulk methylene alkyl chain, 1.45 ppm - β-CH2, 3.35 ppm - α-CH2, 3.55 ppm ethoxylate head. The inset in Figure 5 shows the ethoxylate peaks at δ = 3.55 ppm on an expanded scale. The ethoxylate group spectrum is clearly characterized by six individual peaks, with each peak representing the same number of protons. Thus, each peak 184
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after only the first three EO units. The remaining EO units are hydrated by similar amounts. These results show that the hydration of the micellar shell depends largely on the number of EO units in the head. Moreover, the hydration gradient extends further into the shell as the number of EO units in the head decreases. The existence of the hydration gradient supports our use of a SANS model which does not assume homogeneous mixing in the micellar shell. 3.3.2. NOESY. Two-dimensional nuclear Overhauser effect spectroscopy (2D NOESY) has been shown to be an effective method of studying the conformation of alkyl chains and solubilization in micelles.44,45 The technique can reveal information about the spatial proximity of pairs of coupled protons. Protons with a spatial distance
Influence of Molecular Structure on the Size, Shape, and Nanostructure of Nonionic CnEm Surfactant Micelles
Faheem N. Padia,† Mohammed Yaseen,† Barbara Gore,‡ Sarah Rogers,§ Gordon Bell,∥ and Jian R. Lu*,† †
Biological Physics Laboratory, School of Physics and Astronomy, University of Manchester, Manchester, M13 9PL, U.K. School of Chemistry, University of Manchester, Manchester, M13 9PL, U.K. § STFC ISIS Facility, Rutherford Appleton Laboratory, Didcot OX11 0QX, U.K. ∥ Syngenta, Jealott’s Hill International Research Centre, Bracknell, Berkshire RG42 6EY, U.K. ‡
ABSTRACT: Nonionic alkyl ethoxylates (CnEm) have been extensively studied for their adsorption, aggregation, and solubilization individually and in small groups. In this work, we report a more systematic study of the effects of alkyl chain (tail) and ethoxylate (head) length on the size, shape, and extent of intermixing within the CnEm micelles in aqueous solution. Data from small angle neutron scattering (SANS) and nuclear magnetic resonance (NMR) were combined to undertake the structural characterization of micelles formed from the two separate series of surfactants CnE6 (n = 10, 12, 14) and C12Em (m = 5, 6, 8, 10, 12). The micellar core volume (Vcore) could be well determined with reasonable accuracy and linked to the hydrophilic−lipophilic balance (HLB) of the surfactant, with a sharp size and shape transition occurring around HLB = 12.5. NOESY NMR results revealed protrusions of the terminal methylene groups into the ethoxylate shell, thus providing direct experimental evidence for the phenomenon of “roughness” or intermixing of the core−shell interface. These detailed studies are compared with previous investigations on this model surfactant system.
1. INTRODUCTION There is continuing interest in the relationship between surfactant molecular structure and micellar structure. Such knowledge is fundamental not only for the development of new surfactant based drug delivery systems1,2 and aqueous agrisprays3 but also for the improvement of many existing surfactant based products including cosmetic formulations and cleaning detergents. Insights into the relationship between surfactant molecular structure and micellar structure have already been gained by previous studies. For example, Puvvada and Blankschtein showed that a surfactant’s hydrophobic tail structure has a greater influence on micellar structure than its hydrophilic portion by systematically evaluating the thermodynamic contrbutions from all species involved in micellization.4 With the exception of some early experimental studies on the aggregation numbers of alkyl ethoxylate (CnEm) micelles5,6 and recent studies of their structures by small angle neutron scattering (SANS) for distinct non-ionic surfactants such as C12E6 and C12E12,9,11−13 experimental data to support theoretical findings is generally lacking. Information about the transition of the size and shape of such non-ionic surfactants with alkyl chain length and the size of the ethoxylate head deserves further investigation. Such work can help develop a more quantitative relationship between the molecular structure for the non-ionic series and the micellar structure to be formed. In this work, we have investigated the effects of the alkyl chain length (tail, n) and the number of ethylene glycol units (head, m) on the transition of the size and shape of the non-ionic © 2013 American Chemical Society
micelles using SANS in a more systematic manner. In addition, proton NMR (1H NMR) was used to unravel the detailed hydration of the ethoxylate units and two-dimensional nuclear Overhauser effect spectroscopy (2D NOESY) revealed the conformation of alkyl chains and the spatial proximity of pairs of coupled protons providing useful insight of possible intermixing between alkyl tail segments and the ethoxylate units across the curved interface. These non-ionic surfactants were ideal for the purpose of this study because the lengths of the head and tail groups could be varied independently so that their respective influences on micellar structure could be determined. Moreover, they are among the most commonly used in research and serve as useful models for commercial non-ionic surfactants widely used in industry. Micellization of non-ionic surfactants has been extensively studied to account for the effects of concentration,7 temperature,8 and electrolyte,9 and the influences of addition of surface active additives and mixing with another surfactant.10−13 Although the basic surface packing theory provides a useful outline of micellar shape transition on the basis of surfactant structure, the actual situation is often complex due to the different interactions involved, making it difficult to draw reliable conclusions. For example, Mitchell et al. inferred micellar shapes for a series of CnEm surfactants from macroscopic phase measurements.14 Although the micellar Received: October 2, 2013 Revised: December 2, 2013 Published: December 4, 2013 179
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the same micelles are all oblate or “disk-like” on the basis of sedimentation velocity and intrinsic viscosity data.23 This difference and the model dependent uncertainties inherent of the scattering data analysis suggest that even with the modifications applied to the core−shell model it still may not suitably describe the structure of CnEm micelles. This process must thus be restricted by the sensitivity of the SANS data fitting to the extent of intermixing. To complement the SANS studies in this work, we chose to use an alternative model in which no assumptions are made at the start about the hydration of the head groups and intermixing of the head and tail. The outcome will allow some direct comparison with the previous SANS findings. We have adopted the model developed by Pedersen and Gerstenberg24 and treated the ethoxylate (EO) head groups as individual chains as opposed to a homogenously hydrated mixture.25,26 We will thus need to compare our outcome with the work by Penfold et al. and others to justify the validity of such treatment. With the practical implications of this study to industrial significance, the data is presented with respect to the hydrophile−lipophile balance (HLB), a concept originally introduced by Griffin to correlate the characteristic solution and interfacial properties of non-ionic CnEm surfactants to their molecular structure.27 It was defined empirically and is expressed as
shape predictions were based on a sound geometric-packing theory, two assumptions about the hard-core repulsion between micelles and the order/disorder transition from the isotropic micellar phase to the high concentration phase were difficult to justify. Neutron scattering work by Zulauf et al. later put direct experimental evidence into these systems and showed that these assumptions were not necessarily correct.15 Techniques such as small angle neutron and X-ray scattering (SANS16 and SAXS), dynamic light scattering (DLS),17 and NMR18 provide direct experimental information about different aspects of micellar structures. The scattering profiles from SAXS and SANS contain key structural details about the micelles, but structural parameters about micellar size and shape must be extracted from model analysis or data fitting.19 Although this is a widely adopted and well exercised approach, two or more models may fit the same scattering profile, resulting in different sets of parameters and hence uncertainties of the true micellar structure. In comparison with SAXS, the situation for SANS can be worse because of the narrower momentum transfer range over which data can be reliably determined. However, SANS in combination with deuterium labeling is effective at resolving micellar structures because several scattering profiles can be measured about the same chemical system, thereby providing better structural constraints. Different models have been developed to analyze scattering profiles measured from micellar aggregates. However, they are all based on the common “core−shell” feature with adjustments to account for size and shape deviations. In the representative model for non-ionic CnEm, the core of the micelle is regarded as the homogeneous droplet of the pure alkyl chains and the outer shell contains the respective hydrated head groups. Thus, the whole micelle can be defined from just three parameters, R1, R2, and e, denoting the core radius (which is often constrained to be less than of equal to the length of the fully extended chain), the core + shell radius, and the eccentricity (accounting for the ellipticity of the micellar shape and is equal to R1/R2), respectively. Similar models are widely used to fit SANS data; however, experimental and molecular simulation studies have shown that some of the assumptions made above might not be correct. For example, it is often assumed in the model that there is no intermixing between the core and shell regions but Lu et al. showed from neutron reflection studies on a monolayer of C12E12 that there is a region of intermixing between the alkyl chains and ethoxylate head groups and that the region extends to around 35% of the length of the alkyl chain.20 It is also assumed that the ethoxylate shell is homogenously hydrated but Podo et al. and Majhi et al. showed using 1H NMR21 and single-chain mean field theory,22 respectively, that different extents of hydration existed in the shells of various CnEm micelles. It is often difficult to assess the impact of any incorrect assumptions on the data analysis due to the multiple parameter features, but the core−shell model in its most basic form and modified version does need to be tested and enriched with further structural information from the nonionic surfactant micelles. A number of researchers have overcome the shortcomings of the core−shell model by making certain modifications.13−15 For example, Penfold et al. introduced an additional parameter, ext, to allow for variations in micellar core volumes. The introduction of this new parameter enabled them to fit experimental data for C12E6, C12E8, and C12E12 micelles which were all shown to have prolate or “rod-like” shapes and have advanced from the findings of Tanford et al. who suggested that
HLB =
%mass EO 5
(1)
where %massEO is the mass percentage of the EO head. According to eq 1, the HLB number ranges from 1 to 20 with increasing HLB representing more hydrophilic surfactants. The empirical definition and strong correlation of HLB with surfactant properties has led to HLB becoming one of the most commonly used indicators in industry for determining the suitability of non-ionic surfactants in a given application. In a recent study, Diallo et al. demonstrated that HLB was also a convenient way of systematically ordering a group of surfactants with regard to their micellar properties.28 In this work, we combined data obtained from SANS to elucidate the surfactant molecular structure−micellar structure relationship and NMR to investigate the hydration and intermixing within the micelles.
2. EXPERIMENTAL SECTION 2.1. Materials. Two series of surfactants were purchased from Sigma-Aldrich, CnE6 (where n = 10, 12, and 14) and C12Em (where m = 5, 6, 8, and 10) (≥98%), and were used without further purification. Fully hydrogenated hC12hE12 and chain deuterated dC12hE12 were provided by Dr RK Thomas at University of Oxford. hC12hE12 was prepared by reacting hC12hE6 with hexaethylene glycol (E6) (Fluka, >98%), following the procedures described previously.20 Dried hC12hE6 was first reacted with p-toluenesulphonyl chloride (Fluka, 99%) under dry triethylamine (Aldrich, 99%) to obtain C12E6-tosylate. In a separate flask, dried EO6 in excess was gently mixed with potassium tert-butoxide (Aldrich, 99%) to obtain K-EO6. When the reaction was complete, the sample was mixed with the C12E6-tosylate. The mixture was stirred and heated at 70 °C for 2 h, followed by the addition of 10 mL of hot water to stop the reaction. When cooled, the mixture was neutralized with HCl and extracted with ether. Following solvent evaporation, the raw sample (light yellow oil) was purified through silica flash chromatography. Alkyl chain 180
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S(Q) is the structure factor taking into account the scattering between different particles in the sample, and B is the background signal. For a given sample, the first three terms are constants and in each measurement the background (solvent) signal is reduced from I(Q). Therefore, P(Q) and S(Q) are the only Q-dependent variables. In essence, P(Q) and S(Q) modulate the intensity profile I(Q); thus, I(Q) holds information about all physical parameters of the system. For a dilute system of non-interacting particles such as a low concentration non-ionic micellar solution S(Q) is close to unity. Equation 2 thus simplifies so that I(Q) becomes ∝ P(Q). It thus becomes more straightforward to determine the form (shape and size) of the scattering particle by fitting I(Q). In resemblance to the model developed by Pedersen and Gerstenberg,24 the micellar core is treated as a homogeneous oil droplet consisting of the alkyl chains and the head groups comprised of ethoxylate units adopting the Gaussian chain distribution. Unlike the typical core−shell models used, no assumptions about the hydration of the head groups and intermixing of head and tail groups are made prior to the start of the fitting, as these issues can be easily accommodated by incorporating methylene groups from the tail into the head shell or ethoxylate groups into the tail core during the fitting process. The treatment of short ethoxylate groups as a Gaussian distribution lends its support from the work of Sarmoria and Blanckshtein who showed that even relatively short ethoxylate chains follow standard polymer scaling laws.35 Pedersen and Gerstenberg have derived a number of models for different core structures, but we chose the rotational shape with semiaxes (a, a, c),36 as this treatment was found to be suitable for the CnEm micelles under study and helped reduce the number of parameters used. The model is further outlined below. I(Q) is split into four form factor contributions consisting of the core PC(Q), a single headgroup PH(Q), the interference between head and core PHC(Q), and that between head groups, PHH(Q):
deuterated dC12hE12 was synthesized following a similar procedure as described above. dC12hE6 was made by the Williamson reaction from the deuterated dodecyl bromide, an equimolar amount of sodium, and a 5-fold molar excess of E6.29 All non-ionic surfactants, self-prepared or directly purchased from Sigma-Aldrich, showed single clear spots from our own thin film chromatography (tlc) analysis under a range of concentrations loaded, showing no sign of chain or head mixing. Surfactant solutions were prepared at 100 times their CMCs (critical micellar concentrations) by dissolving the required volume/mass of surfactant in 1 mL of D2O (containing 99.9% D from Sigma-Aldrich). The following CMC values were used:30 0.90 mM for C10E6, 0.067 mM for C12E6, 0.010 mM for C14E6, 0.062 mM for C12E5, 0.10 mM for C12E8, and 0.10 mM for C12E12. For NMR experiments, TSP (3-(trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt) was added to the micellar solutions at a concentration of 1% w/v as a measurement standard. 2.2. Experimental Methods. SANS experiments were conducted on the time-of-flight LOQ diffractometer at the ISIS neutron facility, and the Q-range of 0.009−0.249 Å−1 was used in data analysis. Standard procedures for data treatment were employed.31 Measurements were performed in a 2 mm quartz cell, using a 12 mm diameter beam. The temperature for all experiments was kept constant at 295 K to ensure that samples remained in a single phase throughout the scattering experiment. After data collection completed, the D2O (solvent) scattering profiles were subtracted from beam intensities for background correction. Dynamic light scattering (DLS) was also widely used to study surfactant aggregation.32 This technique helped reveal the hydrodynamic radius, RH, of a scatterer and complements SANS. DLS measurements were performed on a Malvern Zetasizer Nano instrument. Surfactant samples were loaded in quartz cells and left to equilibrate for 5−10 min prior to the measurements. For each sample, three measurements were taken consisting of 15 runs each. Each run lasted 5 s. Values for RH of the micelles were calculated by averaging the three measurements. The temperature for all measurements was kept constant at 295 K. NOESY is highly effective at investigating the internal structure of micelles,33 while 1H NMR can help reveal the extent of hydration of alkyl chain fragments within micelles.34 The 1H NMR and NOESY spectra were recorded in D2O on a Bruker Avance 400 MHz spectrometer. Each spectrum consisted of 16 scans. The gradient NOESY spectra were recorded using a mixing time of 300 ms, and a relaxation delay of 2s and 2k data points was collected for 512 increments of 32 scans. 2.3. SANS Data Analysis. The scattered intensity from a monodisperse sample can be approximated as 2
2
I(Q ) = NV Δρ P(Q )S(Q ) + B
I(Q ) = Nagg 2βC 2PC(Q ) + NaggβH 2PH(Q ) + 2Nagg 2βHβCPHC(Q ) + Nagg(Nagg − 1)βHH 2PHH(Q )
(3)
where Nagg is the aggregation number of the micelle; βC and βH are equal to VC(ρC − ρsolv) and VH(ρH − ρsolv), respectively; Vi and ρi are the volume and scattering length density of an individual chain, respectively; C denotes the hydrophobic core and H denotes the hydrophilic head; and ρsolv is the scattering length density of the solvent, i.e., D2O. Final form factors for PC(Q), PH(Q), PHC(Q), and PHH(Q) are given in eqs 4−8. PC(Q ) =
(2)
∫0
π /2
Φ2[QR(a , ε , α)] sin α dα
(4)
where R(a, ε, α) = r(sin2 α + ε2 cos2 α)1/2, ε = c/a, and
where Q is the momentum transfer, N is the number concentration of the scattering particles, V is the volume of a single scattering particle, and Δρ is the contrast of the scattering particle and is equal to the difference between the scattering length densities of the particle and the solvent, ρs and ρsolv, respectively. P(Q) is the form factor of the scattering particle which introduces interference effects from neutrons scattered from different parts of the same scattering particle,
Φ(QR ) =
3[sin(QR ) − QR cos(QR )] (QR )3
(5)
PH(Q ) =
2[e−x − 1 + x] x2
(6)
where x = Q2Rg2. 181
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Figure 1. (a) SANS intensity profiles measured from C12Em (m = 5 (○), 6 (△), 8 (□), and 12 (◇)) at 100CMC in D2O, with points representing experimental data and curves representing the best fits. (b) For C12E6, the two best fitted curves for different Q regions are given: the dashed curve corresponds to C12E6a and the solid curve to C12E6b as shown in Table 1 to suggest the coexistence of different micellar sizes.
Table 1. Structural Parameters Obtained from the Best Fits Shown in Figure 1 for C12Em (m = 5, 6, 8, and 12) SANS
DLS
surfactant
a (nm ± 0.1 nm)
c (nm ± 0.1 nm)
Vcore (nm )
Nagg
Rg (nm ± 0.05 nm)
Rmax (nm)
RH (nm ± 0.2 nm)
PDI
C12E5 C12E6a C12E6b C12E8 C12E12
1.00 1.50 2.80 1.8 2.0
55.00 8.00 1.25 1.25 1.20
230.4 75.4 46.5 26.5 20.1
710 233 141 80 65
0.45 0.55 0.55 0.60 0.65
55.9 9.1 4.9 3.5 3.3
32.1 5.3 5.3 4.4 4.6
0.231 0.332 0.332 0.101 0.111
3
In the following equations, the short notation R = (R(r, ε, α) will be used for convenience. PHC(Q ) = ψ (QR g)
∫0
π /2
Φ(QR )
sin(Q [R + dR g ]) Q [R + dR g ]
low Q and mid-high Q regions, as illustrated in Figure 1b. This indicates that the scattering profiles can be represented by a single shape of micelles in C12E5, C12E8, and C12E12 solutions, whereas the C12E6 solution may consist of more than one micellar shape. This is discussed in more detail below. Table 1 lists all the fitted parameters for the CnEm micelles studied. The dimensions of the micellar cores are given by the values for a and c. In order to ensure no void existed in the center of the liquid micelle, a was constrained to be ≤ lmax (1.67 nm)the length of the fully extended alkyl chain. For C12E5, C12E8, and C12E12, it can be seen that the a values follow this requirement well, consistent with Tanford’s proposal37 that the shortest dimension of the micellar core is close to 0.8 × lmax ′ , where l′max is the length of the fully extended alkyl chain composed of (n − 1) methylene groups. Tanford proposed that the CH2 group closest to the ethoxylate headgroup protrudes into the shell leaving (n − 1) CH2 groups in the core adopting a conformation with its total length being just shorter than its fully extended length. For a C12 chain, the shortest dimension would, therefore, be 1.22 nm, following Tanford’s estimate. The other dimension, c, was not constrained and was freely fitted. It can be seen that while a remains fairly constant there is a sharp reduction in c with increasing m. In terms of micellar shape, these dimensional changes translate to a shape transformation from prolate ellipsoid for C12E5 to oblate ellipsoid for C12E8. Thus, by increasing the length of headgroup by three ethoxylate units, the micellar cores underwent a prolate-to-oblate shape transformation. For m > 8, there is little further change in micellar shape but there is a slight reduction in the micellar core volume, Vcore. It is useful to comment further on the transformation on the shape of C12E6 micelles. According to the two best fit curves (Figure 1b), two different shapes of micelles, prolate and oblate ellipsoids, coexisted in the C12E6 solution. As the SANS scattering profile represents the average scattering from all species within a sample over an extended period of time
sin α dα (7)
2
PHH(Q ) = ψ (QR g)
∫0
π /2
⎡ sin(Q [R + dR g ] ⎤2 ⎢ ⎥ sin α dα ⎢⎣ Q [R + dR g ] ⎥⎦ (8)
where ψ(x) = [1 − e−x]/x and d ≈ 1, ensuring that the head chains did not protrude into the core since the head chains and core were modeled as two unique structures. This did not imply that intermixing of head and tail groups was not possible. Instead, d was used purely for the mathematical consistency of the model. Overall, the model depends on three fit parameters, a, c, and Rg, where a and c are the semiminor and semimajor axes of the elliptical micelle, respectively, and Rg is the effective radius of gyration of a headgroup chain. Thus, it had fewer fitting parameters than the most core−shell models currently used.
3. RESULTS AND DISCUSSION 3.1. Micellar Nanostructure. 3.1.1. Head Length Effect. SANS scattering results for the surfactant series C12Em (m = 5, 6, 8, and 12) are shown in Figure 1a. For clarity, the SANS profiles are translated in the vertical axis. It can be seen from Figure 1a that there are clear changes in the shape of the scattering profiles as m increases, indicating changes in micellar size and shape. The close fit of the experimental data profiles to the simulated curves achieved using the rotational model as described previously suggests that all four surfactants form ellipsoidal micelles. For C12E5, C12E8, and C12E12, a single set of parameters gave a suitable fit to all the data points, but for C12E6, two separate sets of parameters were necessary to fit the 182
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Table 2. Structural Micellar Parameters Obtained from the Best Fits Shown in Figure 3 for CnE6 (n = 10, 12, and 14) SANS
DLS
surfactant
a (nm ± 0.1 nm)
c (nm ± 0.1 nm)
Vcore (nm3)
Nagg
Rg (nm ± 0.05 nm)
Rmax (nm)
RH (nm ± 0.2 nm)
PDI
C10E6 C12E6b C12E6a C14E6
2.25 2.80 1.50 1.50
1.00 1.25 8.00 20.00
21.2 46.5 75.4 188.5
80 141 233 500
0.55 0.55 0.55 0.55
3.4 4.9 9.1 21.1
3.4 5.3 5.3 18.8
0.066 0.332 0.332 0.207
C12E12, we verify the model analysis based on the ellipsoidal form factor approach by first deriving micellar shape changes following a consistent trend as outlined in Table 1 and second by fitting the derived micellar parameters for C12E12 to a second set of SANS profiles measured under different isotopic contrasts. Figure 2 shows the experimental SANS scattering profiles of the fully protonated C12E12 (isotopic composition hC12hE12)
(approximately 1 h), the data indicates that the two groups of micellar populations are in equilibrium. The possible coexistence of different but similar micellar shapes has also been suggested by Gapinski et al. They commented that while they could use the elliptical rod-like structure as an overall representative model, their data could also be well analyzed using a similar approach. The coexistence of different micellar shapes is actually not new, as Nagarajan had predicted the coexistence of micellar populations over a decade ago.38 However, only recently has there been evidence to support his claim. For example, simulations performed with the recently developed single-chain mean field theory suggest that coexistence of multiple populations is quite common.22,39 Work by Glatter et al.40 and another study by BernheimGroswasser et al.41 using cryo-TEM imaging also led to evidence for coexisting micellar populations, showing spherical and thread-like micelles from their non-ionic surfactant solutions. The polydispersity index values, PDI, obtained from DLS measurements also support these observations. The PDI indicates the distribution of correlation functions (which are used to calculate RH) measured in a sample.42 For a monodisperse sample, the distribution of correlation functions is small and hence the PDI value is small. On the contrary, a polydisperse sample will suffer from a large distribution resulting in a high PDI number. As shown in Table 1, the PDI numbers for C12E8 and C12E12 are small, indicating that these micelles are rather monodisperse. For C12E5 and C12E6, however, the PDI numbers are much larger, suggesting a higher degree of polydispersity associated with different sizes or dimensions in the micellar systems. The volume of the micellar core, Vcore, was calculated from the core dimensions for each micelle. It can be seen that the shape transformation is accompanied with an approximately 10fold reduction in volume. Additionally, there is a small increase in Rg with m, which suggests that, in spite of the large transformation of core shape and size as a result of the growth of the headgroup, the conformation of the headgroup does not change significantly. Nagg, the average micellar aggregation number was calculated by dividing Vcore by Vchain, the volume of a single hydrocarbon chain, as given by Tanford’s expression Vchain = 27.4 + 26.9(nc − 1)
Figure 2. SANS scattering profiles for C12E12 at 100CMC under different isotopic contrasts, hC12hE12 (◇) and dC12hE12 (×). Points represent experimental data, and curves represent simulated best fit curves. The same structural parameters fit both sets of experimental data, adding confidence to the model proposed.
and chain deuterated C12E12 (dC12hE12) as an example, with the calculated curves of the best fits also shown. The best fit curves are produced using the same ellipsoidal model and parameters (see Table 1) for both isotopic contrasts. As the isotopic composition of the surfactant has no effect on the micellar nanostructure but does change the scattering profiles, the close fit of the calculated curves to the experimental data as illustrated in Figure 2 verifies the ellipsoidal model and the obtained parameters. The hydrodynamic diameter, RH, was also measured for each micellar solution using DLS, with the measured values listed in Table 1. In DLS measurements, RH is calculated from the diffusion constant, D, of a particle using the Stokes−Einstein relation which is given by
D=
(9)
where nc is the number of carbons in the chain. It can be seen that Nagg decreases gradually with m. This is in agreement with previous experimental data that showed an almost linear decrease in aggregation number with head length (see Tables 1 and 2).5,6 The Nagg values for C12E6b and C12E8 are 141 and 80, respectively, which is in close agreement with those reported by Zulauf et al., 140 and 95, respectively.15 On the other hand, the Nagg value of 65 for C12E12 is lower than that of 79 reported by Penfold et al.13 It is useful to check the reliability of the Nagg values calculated so that the discrepancies could be tested. For
kT 6πηRH
(10)
where k is the Boltzmann constant, T is the temperature in K, and η is the viscosity of the solvent. The relation in eq 10 is only true for spherical particles. For non-spherical particles, the value of RH does not relate to the actual radius of the particle, but instead it gives a value that is equivalent to the radius of a spherical particle that would diffuse with the same diffusion constant as the scattering particle in the sample. As such, the measured RH value is only used here as an indication of the order of magnitude of the micellar dimensions. It can be seen that RH is similar to Rmax (the biggest dimension obtained from 183
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C12E5) or whether this is due to the dynamic nature of micelles which inherently leads to greater polydispersity in larger micelles. The RH values for all the CnE6 surfactants are of the same order of magnitude as Rmax, thus broadly consistent with the dimensions obtained from SANS modeling. 3.2. Hydrophile−Lipophile Balance (HLB). As HLB is widely used as a measure of changes in amphiphilicity, we also examined how the shape and size changes were affected by it. This is done using the HLB, as shown in Figure 4, where
SANS) for all C12Em micelles, consistent with the physical implication of the hydrodynamic dimension. 3.1.2. Chain Length Effect. Changes in the scattering profiles as a result of increasing the alkyl chain length for CnE6 (n = 10, 12, and 14) are shown in Figure 3, with an isotopic composition of hC12hEm in D2O.
Figure 3. SANS intensity profiles for CnE6 (n = 10 (◇), 12 (□), 14 (△)) micelles at 100 times cmc in D2O. Points represent experimental data, and curves represent the best fits using the triaxial model. For C12E6, the two best fitted curves to different Q regions are given: the solid curve corresponds to C12E6a in Table 1, and the dashed curve is C12E6b.
Figure 4. Changes in micellar core volume with HLB for the C12Em series (◆) and the CnE6 series (●), with shape transitions also schematically represented. There is a sharp drop in micellar core volume around HLB = 12.5 where different shapes of micelles might coexist.
The micellar parameters obtained from the data fit are listed in Table 2. The two sets of fitted parameters for C12E6 have been included in reverse order in Table 2 to illustrate the shape transformation with increase in n. The transformation is opposite to that occurring when m is increased. When n = 10, the micelles are small and oblate, but for n = 14, the micelles are much longer and prolate in shape. The overall shape transformation is thus oblate-to-prolate when n increases. Although this shape transformation is predicable, it has not been shown for these alkyl ethoxylate micelles. A similar transformation was observed by Arleth et al. in a mixed system of egg yolk phosphatidylcholine (PC) and polyethylene glycol (PEG) modified distearoyl phosphatidylethanolamine (DSPEPEG).43 DSPEPEG had a bulkier headgroup, and the mixed micelles were found to transform from oblate to prolate ellipsoid as the mole fraction of PC increased from 0 to 0.4. Thus, the preference for prolate micelles when the hydrophobicity of the surfactant(s) is increased is consistent in both systems. Table 2 also shows that Rg is constant with n, suggesting that there was little conformational change of the head groups as the alkyl chain was increased. In the previous section, we saw about 10-fold reduction in core volume when the number of ethoxylate units in the headgroup was increased from m = 5 to 12. In contrast, Table 2 shows nearly a 10-fold increase in core volume as the chain length is increased from n = 10 to 14. Despite the similar magnitude of the volume change, the relatively small change in tail length has had a much greater influence on the micellar core volume than the head length, thus supporting the theoretical prediction by Puvvada and Blankschtein.4 Table 2 shows a similar trend in PDI values as seen in the previous section. The small oblate C10E6 micelles show low polydispersity, while C12E6 exhibits a much higher PDI value due to the coexistence of multiple micellar populations. It is not clear from this data, however, whether the high PDI value for C14E6 is also due to coexistence of micellar populations (as with
micellar core volumes are plotted against HLB with the corresponding micellar shapes overlaid. The squares represent surfactants in the C12Em series, and the diamonds denote the CnE6 series. The results shown in Figure 4 indicate a rapid reduction in micellar volume as HLB increases when HLB < 12. The reduction in volume is accompanied by a prolate-to-oblate shape transformation. At around HLB = 13, changes in size and shape plateau and further increase in HLB has no effect on micellar shape, although a slight reduction in volume occurs, demonstrating a general relationship for the non-ionic surfactants. From Figure 4, it can be seen that the coexistence of different micellar shapes in C12E6 coincides with the shape and size transition at around HLB = 12.5. To alleviate any potential concerns that the presence of the two shapes arose due to impurities in the surfactant, thin layer chromatography and NMR were both performed on the C12E6 samples. No impurities were found. 3.3. Shell Structure and Hydration. To complement the SANS study, we also examined the internal structure of the micelle and the hydration of the micellar shell using 1H NMR and two-dimensional nuclear Overhauser effect spectroscopy (2D NOESY). 3.3.1. 1H NMR. The chemical shifts of protons in the C12E6 micellar solution are shown in Figure 5. The contribution to the NMR spectrum from monomers is assumed to be negligible as the sample concentration was much greater than the CMC. The peaks are identified as follows: 0.78 ppm - terminal methyl group in alkyl chain, 1.2 ppm - C9H18 bulk methylene alkyl chain, 1.45 ppm - β-CH2, 3.35 ppm - α-CH2, 3.55 ppm ethoxylate head. The inset in Figure 5 shows the ethoxylate peaks at δ = 3.55 ppm on an expanded scale. The ethoxylate group spectrum is clearly characterized by six individual peaks, with each peak representing the same number of protons. Thus, each peak 184
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after only the first three EO units. The remaining EO units are hydrated by similar amounts. These results show that the hydration of the micellar shell depends largely on the number of EO units in the head. Moreover, the hydration gradient extends further into the shell as the number of EO units in the head decreases. The existence of the hydration gradient supports our use of a SANS model which does not assume homogeneous mixing in the micellar shell. 3.3.2. NOESY. Two-dimensional nuclear Overhauser effect spectroscopy (2D NOESY) has been shown to be an effective method of studying the conformation of alkyl chains and solubilization in micelles.44,45 The technique can reveal information about the spatial proximity of pairs of coupled protons. Protons with a spatial distance
